1. Technical Field
The invention relates to a non-aqueous electrolyte liquid for a secondary battery, and a lithium secondary battery containing the electrolyte liquid. More particularly, the invention relates to a non-aqueous electrolyte liquid for a secondary battery containing a particular silicon compound, which is suitably used in lithium secondary batteries, and a lithium secondary battery using the electrolyte liquid.
2. Background Art
Lithium secondary batteries are used in PCs, video cameras, mobile telephones, and the like. Along with a functional enhancement of these electronic instruments, an increase in the energy density is desired in relation to the batteries that serve as power sources. Furthermore, in recent years, against the background of a global-scale environmental problem of the so-called reduction of carbon dioxide emissions, size increment of lithium secondary batteries has also been under consideration in connection with the applications of automotive power supplies or storage of natural energy, and there is an increasing demand for improvements in cost, performance, and safety. Thus, there is a demand for the development of an electrolyte liquid which enables such improvements.
Regarding the performance of large-sized batteries, particularly an enhancement of energy density (electric capacity per unit mass or unit volume) has been a great task. One of the effective methods for such an enhancement involves increasing of the battery voltage. In order to increase the battery voltage, it is essential to use a positive electrode material having a high lithium insertion/release potential, and an electrolyte liquid having an oxidation resistance to withstand the potential.
In conventional electrolyte liquids, use has been made of solvent mixtures of a cyclic carbonate such as ethylene carbonate (EC) or propylene carbonate (PC) and a chain-like carbonate such as diethyl carbonate (DEC) or dimethyl carbonate (DMC) for decreasing the viscousness of the electrolyte liquid. However, since these solvents do not have sufficient oxidation resistance, when a high voltage positive electrode is used, there is a problem that decomposition of the solvent occurs, and the battery voltage is decreased.
As a means for increasing the voltage resistance of such an electrolyte liquid as described above, two methods have been suggested, such as (1) a method of using a solvent having a molecular structure that is itself not easily oxidizable, as a solvent; and (2) a method of adding a small amount of an additive for suppressing oxidative decomposition of solvent molecules by acting on the surface of the electrode material, to a conventional solvent.
Examples of the solvent having a molecular structure that is not easily oxidizable, which have been suggested as solvents that can be used in non-aqueous electrolyte liquids for secondary batteries, include sulfolane as suggested in JP 6-223874 A; dimethyl malonate as suggested in JP 8-162154 A; diethyl malonate as suggested in JP 8-190932 A; thiocyanic acid esters as suggested in JP 9-171839 A; and halogen-substituted carbonates as suggested in JP 10-144346 A.
However, even if conventionally known solvents that are not easily oxidizable are used, it is difficult to obtain oxidation resistance at a practically sufficient level, and it is the current situation that a solvent which is capable of achieving a balance between ion conductibility and oxidation resistance has not been found yet. Furthermore, using a method of using an additive that suppresses oxidation of solvent molecules can also be taken into consideration; however, there is a defect that the amount of the additive that can be optimally used is limited, and the voltage resistance effect is not sustained because an increase in the amount of addition causes a decrease in the battery performance when such an additive is added in an amount sufficient for oxidation resistance.
In addition, JP 8-88023 A suggests an electrolyte liquid having self-extinguishability and satisfactory charging-discharging performance, which contains a phosphoric acid ester compound as an electrolyte in a hydrocarbon-based solvent, and a battery using the electrolyte liquid. However, this battery still cannot be said to be satisfactory for practical use in terms of flame retardancy, and a further improvement is needed.